Semiconductor

 Semiconductor :-The conductivity lies in between the conductors and insulators. The energy gap is narrow which is equal to one electron volt. In case of semiconductors in order to conduct current i.e. to jump electrons from valence band to conduction band room temperature is sufficient hence for this they are mainly used for industrial and domestic applications. The examples of semiconductors are germanium (Ge) and silicon (Si).

Intrinsic Semiconductor :-Intrinsic semiconductor is a pure form of semiconductor this means the number of electrons present in the material are equal to number of holes. 

Extrinsic Semiconductor :-When impurities are added to the semiconductor then it is called extrinsic semiconductor. In this type of semiconductor number of electrons are not equal to number of holes as they vary depending upon the dopants added.

                 Periodic Table :-

P- type Semiconductor :-When trivalent impurities such as gallium, indium, and bismuth are added to a pure semiconductor a p-type semiconductor is formed. The addition of these impurities makes the number of holes greater than the number of electrons hence the name p-type. This type of impurities are called acceptor impurities because the holes formed can accept the electrons.

The formation of p-type semiconductor can be explained using the diagram shown below.

Consider a Germanium crystal shown in the figure. Now a Dopant such as Gallium is added to this Germanium. The Gallium is trivalent impurity i.e. it consists of three valence electrons. These 3 electrons are shared with Germanium atom and three covalent bonds are formed. But remaining atom of Gallium has no valence electron to share hence one empty hole is created.

P- type Semiconductor :-

N- Type Semiconductor :-When a small amount of Pentavalent impurities like arsenic, antimony and phosphorus are added to the germanium semiconductor an n -type semiconductor is formed. In an n-type semiconductor number of electrons are more than the number of holes hence the name n-type. The added impurities for n-type semiconductor are called donor electrons.

Consider a germanium crystal and a Pentavalent impurity arsenic this can be shown in the figure.

The Pentavalent impurity arsenic has 5 atoms, these are shared with 4 atoms of germanium semiconductor forming covalent bond. The remaining electron is the extra free electron. Hence by adding arsenic more number of electron pairs can be created.

N- type Semiconductor :-

PN Junction Diode:-• P-type and N-type materials by themselves are of little use. When a piece of P-type material is suitably joined to a piece of N-type material, a P-N junction formed. Such a PN-junction makes a very useful device and is called a semiconductor diode.

• A PN-junction cannot be made by simply joining the two pieces together. Special fabrication techniques are needed to form a PN-junction.

•The various methods of fabricating PN-junction are growth and alloying techniques. Generally it is not required to study the fabricating methods of P-N junction at this stage. In the below diagram the arrow mark indicates the direction of conventional current direction in the diode, i.e., from P-type to N-type. An important characteristic of a P-N junction is its ability to conduct current in one direction only. In reverse direction it offers very high resistance.

PN Junction Diode :-

PN Junction Diode with forward bias :-•If the positive terminal of the external voltage source (V) is connected to the P side and negative terminal to the N side of the P-N junction then the diode is to be in forward biased mode.

•Forward biased P-N junction permits easy flow of current across the junction. The current flow may be explained in the following two ways.

•As soon as battery is connected, holes are repelled by positive terminal of the battery and the electrons are repelled by negative terminal of the battery there by holes and electrons are driven towards the junction where recombination process takes place. The movement of electrons to the left and holes to the right of the junction constitutes a large current flow through the semiconductor.

 •Obliviously, the crystal offers low resistance in the forward bias.•Another way to explain current flow in the forward bias is to say that due to the applied external voltage, the width of depletion layer is reduced and there by the height of the barrier potential also reduces allowing to flow across the junction.

PN Junction Diode with forward bias :-

PN Junction Diode with reverse bias:-

•If the positive terminal of the external voltage source (V) is connected to the cathode (N side) and negative terminal to the anode (P side) of the P-N junction, the diode is said to be in reverse biased mode. In this case, holes are attracted by the negative terminal of the voltage source and electrons by the positive terminal so that both holes and electrons move away from each other. 

•Since there is no electron hole recombination no current flows and hence the junction offers high resistance.

•The applied voltage increases the width of the depletion layer and there by the height of the barrier potential also increases making less amount of current to flow through the junction.

PN Junction Diode with reverse bias :-

Semiconductors are mainly classified into two categories :-

1. Intrinsic semiconductor.

2.Extrinsic semiconductor.

Semiconductor :-

Semiconductors materials such as silicon (Si), germanium (Ge) and gallium arsenide (GaAs), have electrical properties somewhere in the middle, between those of a “conductor” and an “insulator”. They are not good conductors nor good insulators. The amount of impurities added to the semiconductor material it is possible to control its conductivity.

These impurities are called donors or acceptors depending on whether they produce electrons or holes respectively.

The process of adding impurity atoms to semiconductor atoms  is called Doping.

Intrinsic semiconductor :-An intrinsic semiconductor material is chemically very pure and possesses poor conductivity. It has equal numbers of negative carriers (electrons) and positive carriers (holes).

Extrinsic semiconductor :-An extrinsic semiconductor is an improved intrinsic semiconductor with a small amount of impurities added by a process, known as doping, which alters the electrical properties of the semiconductor and improves its conductivity.

Doping process produces two groups of semiconductors: the negative charge conductor (n-type) and the positive charge conductor (p-type). Silicon and Germanium are the most common elemental semiconductors.Most of the semiconductor devices and chips are created with silicon.

Carbon, silicon and germanium have a unique property in their electron structure -- each has four electrons in its outer orbital. This allows them to form nice crystals.The four electrons form perfect covalent bonds with four neighboring atoms, creating a lattice.

A pure silicon crystal is nearly an insulator  very little electricity will flow through it. You can change the behavior of silicon and turn it into a conductor by doping it.

The most commonly used semiconductor basics material by far is silicon. Silicon has four valence electrons in its outermost shell which it shares with its neighbouring silicon atoms to form full orbital’s of eight electrons. The structure of the bond between the two silicon atoms is such that each atom shares one electron with its neighbour making the bond ve

 Diode :-

A diode is a two-terminal electronic component.It has low  resistance to current in one direction, and high resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Today, most diodes are made of silicon, but other semiconductors such as selenium or germanium are sometimes used.

Forward Biased PN Junction Diode:-